I get your point that a node in the network can transfer a different packet. But I think people will have to be more creative with the protocol. For example, other nodes can hear both your transmission and next hop and can detect the packet was changed. I'm not sure you could make it 100% reliably, but the network could be self policing. Hopefully it would be one of those things that doesn't work in theory, only in practice. (And if not, then the rules would have to be updated.)